The present invention relates to nuclear magnetic resonance phenomena and in particular to the use thereof in spectroscopic analysis.
The on-line analysis of chemical composition of samples undergoing continuous translational motion, for example through pipes or on conveyors is important throughout the food, chemical and pharmaceutical industries. To date, a number of spectroscopic and physical methods are used for on-line compositional analysis, including near infra-red spectroscopy (NIR), microwave sensors and ultrasonics.
However Nuclear Magnetic Resonance (NMR) has not hitherto been exploited in such an on-line mode on continuously moving objects, despite its well established analytical role in the off-line analytical laboratory. A possible exception to this statement is its use in the on-line analysis of slowly flowing liquids and pastes (C. Tellier and F. Mariette, Online applications in Food Science, Annual Reports on NMR spectroscopy, Vol. 31, 1995, 105-122).
However such applications use conventional NMR methodology and hardware and the fluid velocity is therefore limited by the restricted volume over which the magnetic field and/or radiofrequency field is homogeneous.
Another previous on-line analytical application is in the analysis of powders by NMR (C. I. Nicholls and A. De Los Santos, Hydrogen transient NMR for industrial moisture sensing, Drying Technology, vol 9, 1991, 849-873). But this is based on removing a sample of powder (e.g. with a piston arrangement) for analysis while it is stationary, using conventional NMR methods. There are several reasons for the failure to exploit NMR in a general way for on-line analysis of continuously moving samples.
First, most information is obtained in an NMR spectrum when the spectral peaks arising from different molecular species are well resolved. The spectrum is then easier to interpret and the spectral peak areas, which are related to the number of spins contributing to the peak, can be obtained from each resolved peak by simple integration. This has meant that, in off-line analytical laboratories, it is preferable to work with high (i.e. strong) magnetic fields, since the peak separation increases with increasing field strength. Unfortunately, creating high, spatially uniform magnetic fields (eg. above proton resonance frequencies of 100 MHz) requires the use of superconducting magnets. These magnets are very expensive and require regular filling with liquid nitrogen and liquid helium and are not therefore suitable for routine exploitation and use on the factory floor. If lower magnetic fields are used, such as those available with low-cost, robust, permanent magnets (with proton resonance frequencies less than approximately 20 MHz), the spectral peaks from different molecular species are not usually resolved in the NMR spectrum, making compositional analysis of the spectrum more complicated. This is probably one reason why low field NMR spectra have not yet been exploited for compositional analysis.
Another reason for the slow exploitation of conventional NMR in an on-line mode is the requirement of field homogeneity, both in the main magnetic field, B0 and in the radiofrequency field, B1. Conventional hardware for generating spatially uniform radiofrequency fields based, for example, on Helmholtz coils, saddle coils and birdcage coils creates uniform radiofrequency radiation only over a limited volume. The NMR spectrum of the moving sample must therefore be acquired while the sample resides inside this limited volume. This severely limits the sample velocity and means that most NMR analysis is performed either in a side-line mode where the sample velocity is slowed, or in an off-line mode on stationary samples.
It is an object of the present invention to provide a method of obtaining magnetic resonance spectroscopic data and deriving chemical composition data therefrom.
It is a further object of the present invention to provide an apparatus which can maintain homogeneous B0 and B1 (RF) fields over a sufficient length of travel of an object undergoing translational motion to enable the collection of spectroscopic nuclear magnetic resonance data for determining chemical composition of the object.
According to one aspect, the present invention provides a method of nuclear magnetic resonance spectroscopy comprising the steps of:
conveying an object to be analysed through an NMR module at a predetermined velocity, v;
generating, within the NMR module, a spatially characterised, constant magnetic field B0 substantially parallel to the direction of the velocity, v;
generating, within the NMR module, a radiofrequency field B1 pulse transverse to field B0;
detecting nuclear magnetic resonance signals weighted with at least one selected nuclear magnetic resonance parameter from said object;
generating a chemical shift resolved spectrum of the conveyed object within the NMR module.
According to a further aspect, the present invention provides apparatus for gathering spectroscopic nuclear magnetic resonance data comprising:
a first field generating means for generating a spatially characterised, constant magnetic field B0 in an NMR module volume having a predetermined length along a longitudinal axis thereof, the B0 field being parallel to said longitudinal axis;
a second field generating means for generating, within the NMR module volume, radiofrequency field B1 pulses transverse to field B0;
receiver means for detecting nuclear magnetic resonance signals weighted with at least one selected nuclear magnetic resonance parameter from said object;
means for generating a chemical shift resolved spectrum of the conveyed object within the NMR module;
wherein at least said second field generating means comprises a coil having cylindrical geometry.